The architectural organization and signal processing functions of the human brain and nervous system are distinctly different and offer particular advantages over conventional, von Neumann digital electronic (computer) systems for a number of important applications, such as pattern recognition for speech and image analysis, and content addressable memory. The computational capabilities of these biological neural systems arise from the collective interaction of many neurons (signal processing elements) which are massively interconnected in a complex network. While the characteristic time associated with individual neuron operation is of the order of a millisecond, neural systems can outperform digital systems for certain applications, even though the characteristic operation time for current digital computer elements is of the order of nanoseconds. Current research in developing artificial neural systems is primarily directed along two lines: (1) simulating in software running on conventional digital computers the signal processing algorithms employed in neural systems, and (2) the development of custom hardware to implement the neural systems approach to signal processing. The first approach suffers from the problem of simulating the largely parallel, asynchronous operation of neural systems on a serial, synchronous computer. The second suffers from inefficiency in replicating the operation of the neuron and its interconnections.
While biological neural systems use a pulse-mode form of signal transmission, wherein information is communicated by varying the frequency of pulses, most approaches to neural hardware implementation utilize conventional digital circuit architecture (digital approach), wherein the information is transmitted and processed in binary form, or use conventional analog circuits (analog approach) such as operational amplifiers where the information is processed as an analog voltage or current. In a few instances the neuron's pulse-mode operation has been replicated, but only with relatively complicated circuits. However, in virtually all of these implementations, conventional transistor-based circuits have been employed so that the electronic equivalent of an individual neuron is a rather complex, multi-transistor circuit. This fact poses significant barriers to the development of complex neural systems employing large numbers (thousands to millions) of electronic neurons, including limitations on integration due to power, area, interconnections, cost and reliability.
The more conventional approach, based on transistors such as silicon MOSFET types and employing standard circuit configurations, typically requires tens to hundreds of transistors plus additional devices per neuron so that the device area is correspondingly quite large. This translates into higher cost and increased complexity in design, and also limits the number of artificial neurons that can be integrated into a monolithically integrated system. This is important in artificial neural systems where the power of the neural approach is achieved by the simultaneous operation of large numbers of relatively slow individual neurons. The drawback in using GaAs-type semiconductor materials is that the technology for compound semiconductor device fabrication is not as well developed (and as inexpensive) as for silicon, although it is certainly adequately advanced so that commercial devices, e.g. semiconductor lasers, are readily available.
A unique approach to mimicking the pulse-mode operation of the neuron in developing a custom artificial electronic neuron is disclosed in Semiconductor Electronic Concepts for Neural Network Emulation, by D. D. Coon and A. G. U. Perera, Int. J. Electronics (1987), in which a p-i-n semiconductor diode is operated as a switching element in a relatively simple set of circuits. The operation in these circuits replicates a number of important characteristics of the biological neuron, including threshold behavior, temporal integration, memory, synaptic weighting, excitation and inhibition, summation over multiple inputs, and distribution to multiple outputs. However, a substantial barrier to its implementation is its requirement of extremely low temperatures (4.2.degree. K. or -269.degree. C.) for operation of the p-i-n diode. Such low temperatures preclude integration of the device with other conventional devices such as resistors and transistors, and add considerable cost and complexity to its utilization, making this approach impractical.
In the approach of Coon and Perera to mimic the pulse-mode operation of the biological neuron, a p-i-n semiconductor diode is operated as a switching element in a relatively simple set of circuits. The starting point of the switching mechanism is the fact that electrons are trapped at their parent donor atoms in the weakly n-type i-region of the device while in the low conductance mode. When the bias is raised sufficiently, electrons injected into the i-region from the cathode are accelerated and gain sufficient energy to produce impact ionization of trapped electrons from these parent donors. An avalanche multiplication of mobile electrons results, with each electron freed by this impact ionization process leaving behind a positively charged, immobile donor atom. The buildup of this positive charge in the i-region enhances the cathode electric field which ultimately accounts for the switch into a high conductance mode of device operation. The avalanche multiplication process is thus responsible for the device switching from the low to high conductance state.
While the above described mechanism is used by Coon and Perera to produce a p-i-n diode which can switch between low and high conductance current modes, a substantial barrier to its practical implementation is its requirement of extremely low temperatures (4.2.degree. K. or -269.degree. C.) for operation as described above. These very low temperatures are required to ensure that electrons are initially trapped at the donor atoms in the i-region, since the depth of the potential energy well (binding energy) for the electrons is only approximately 25 meV. To ensure binding of electrons at the donors, the mean thermal energy (which is approximately=kT--Boltzmann's constant times temperature) must be much less than the binding energy. For such shallow potential energy wells, switching operation is only possible at extremely low temperatures. At 4.2.degree. K. the mean thermal energy is kT=0.36 meV, but the mean thermal energy at room temperature is kT=26 meV. Such low temperatures (4.2.degree. K.) preclude integration of the device with other conventional devices such as resistors and transistors since this same effect (electron trapping) interferes with normal device operation. Hence, implementation of Coon and Perera's approach based on the low temperature operation of the p-i-n diode adds considerable cost and complexity to its utilization, thereby making this approach impractical.
By comparison, to overcome the low temperature limitations of Coon and Perera's p-i-n diode, a practical diode can be constructed having multiple quantum wells. To construct such a practical diode which can be used in a circuit to emulate a biological neuron, the depth of the quantum wells used to confine the electrons in the low conductance mode should be of the order of hundreds of meV (e.g., 300 MmeV), which is more than an order of magnitude larger than the binding energy of the donors in the i-regions of Coon and Perera's device. As a result, the operation of such a practical multi-quantum well injection mode diode (multi-well diode), can occur at much higher temperatures, including room temperature where the mean thermal energy is approximately kT=26 meV.
Another reference, Tunneling-assisted impact ionization for a superlattice, by Chuang and Hess, J. Appl. Phys. (15 Feb. 1987), discloses an avalanche photodiode which consists of a multi-quantum well structure having several alternating layers of undopeal, wide-gap semiconductor and heavily doped, n-type narrow-gap semiconductor layers, having about equal thicknesses. This multi-layer structure is sandwiched between an n+ cathode and a p+ anode which are formed with a narrow-gap semiconductor. Ohmic contacts made to the n+ quantum wells are needed so the avalanche photodiode can replace electrons lost to impact ionization, to maintain the sensitivity of the device to incoming light.
The avalanche photodiode of Chuang and Hess is designed to detect light (photons), and to produce an output electrical signal (current pulse) proportional to the intensity of the incoming light or rate of arrival of photons. In contrast, a practical multi-well diode used to emulate operation of a biological neuron preferably is designed for use in the dark, and to produce an electrical output pulse, wherein its operation in various circuit configurations would be designed to produce a set of output pulses of essentially uniform height and shape, with the output information carried by the frequency of the pulses.
The avalanche photodiode of Chuang and Hess is operated in reverse bias. When the avalanche photodiode is operated at high reverse bias in the dark, minimal leakage current is desired which is constant in time and as small as possible. Since the applied bias is sufficient to produce the avalanching effect, the injection of electrons into the multi-quantum well region must be held to practically zero to prevent premature (in the absence of light) avalanching. To accomplish this the reverse bias mode is used. If the reverse leakage current is not small enough (even temporarily), then avalanching may occur even in the absence of light, producing an unwanted output electrical pulse. These undesirable electrical pulses thus constitute noise in the output signal which can mask the detection of true, optically generated signals.
A practical multi-well diode and the avalanche photodiode of Chuang and Hess are similar in construction, both employing multiple quantum wells and employing the avalanche effect. The avalanching effect in the Chuang and Hess photodiode, however, is not desirable except in the presence of light. While undesirable spontaneous avalanches in the absence of light cannot be completely eliminated in the avalanche photodiode of Chuang and Hess, in contrast, such spontaneous avalanches in the absence of light are the desired effect in a practical multi-well diode used to emulate operation of a biological neuron.
The avalanche photodiode of Chuang and Hess has certain similarities to that required in producing a practical multi-well diode which can be used to emulate a biological neuron. The avalanche photodiode and the multi-well diode, however, are constructed somewhat differently, are biased in a different fashion in their operation, and are employed for different purposes. The avalanche photodiode, as related above, requires ohmic contacts to its n+ quantum wells, whereas the practical multi-well diode does not require such contacts. In fact, such contacts would preclude the proper operation of the multi-well diode in switching to the high conductance mode, since such contacts would prevent the formation of positive space charge in the quantum wells.
The purpose of the avalanche photodiode of Chuang and Hess is to detect photons and produce an electrical output that is proportional to the rate of arrival of the photons. The purpose of a practical multi-well diode is to emulate the operation of a biological neuron in a mode of operation which is not dependent upon the multi-well diode directly receiving any given amount of optical energy.
The avalanche photodiode of Chuang and Hess is operated in reverse bias, whereas a practical multi-well diode is operated in forward bias to encourage electron injection from the cathode to initiate the avalanching effect. The operation of the multi-well diode is similar to the parasitic breakdown mode of operation of the avalanche photodiode.
A further reference, Mechanism of an S-shaped current-voltage characteristic in a Multilayer Isotypic GaAs--AlGaAs heterostructure, by Alferov et al., Sov. Phys. Semicond. (March 1987), discloses a multi-hetero-structure having an S-type current-voltage characteristic, which is associated with electron transport across alternating lightly doped, n-type AlGaAs barriers and heavily doped, n-type GaAs layers. A theoretical description of the negative resistance phenomena is given based on collective heating of electrons in the GaAs layers by electrons accelerated in the AlGaAs barriers. The weak temperature dependence observed in the current-voltage characteristics is explained wherein electron tunneling through the AlGaAs barriers is the dominant transport mechanism in the AlGaAs barrier layers. Experimental results are presented which show a weak negative differential resistance region having a threshold voltage of 6-7 volts, and a holding voltage of 5-6 volts.
The threshold and holding voltages of a practical multi-well diode used to emulate operation of a biological neuron are preferably much lower than the 6-7 volts and 5-6 volts disclosed in the above Alferov et al. reference. Biological neurons operate with voltage pulses in the range of several hundred millivolts, and a practical multi-well diode circuit which is to be interfaced with actual biological neurons would have to operate within the same voltage range. In addition, the lower the voltage required for operation, the lower the power required to supply such a practical multi-well diode circuit, and the closer such circuits could be packaged together to make integrated circuit packages having multiple practical multi-well diode circuits.